Sodium dithionite is the strongest sulfur-based reducing agent known. It has a number of industrial uses, including the "bleaching" of textiles, paper and clay. Syntheses of this chemical have been known since the 19th century and include electrolytic means. The electrolytic methods generally involve the reduction of bisulfite (HSO.sub.3 -) to produce either zinc dithionite or sodium dithionite, and can be done using cells of various kinds. These cells are in some cases compartmented and employ electrodes of various materials. For example, U.S. Pat. No. 4,144,146 discloses a process for the production of dithionite by cathodic reduction of an aqueous solution in a compartmented cell using a cathode made of a noble metal, electrically conductive noble metal oxide, silver, chromium, stainless steel, or any of various other metals and alloys. The use of a graphite cathode in a compartmented cell having a cation-active permselective membrane is mentioned in U.S. Pat. Nos. 3,920,551 and 3,905,879, but is considered undesirable for a variety of reasons, including mechanical instability.
A problem encountered in electrolytic dithionite production is the decomposition of the product. In general the zinc dithionite is more stable than the sodium dithionite with respect to anaerobic decomposition. However, zinc dithionite is seldom used now because of environmental concerns. In contrast, sodium dithionite decomposes easily. This decomposition can take place both aerobically and anaerobically. The aerobic mechanism involves the diffusion-controlled reaction of oxygen with dithionite. Sodium dithionite (Na.sub.2 S.sub.2 O.sub.4) decomposes by this pathway to ultimately form sodium sulfate (Na.sub.2 SO.sub.4). Anaerobic decomposition involves the reaction of the dithionite to form sodium bisulfite (NaHSO.sub.3) and sodium thiosulfate (Na.sub.2 S.sub.2 O.sub.3) via a disproportionation mechanism. Sodium thiosulfate formed by the anaerobic decomposition of sodium dithionite is an undesirable product in many cases. This is because it is very corrosive to metals in general, and even to some stainless steel. This corrosiveness presents particular problems in paper mills because it damages suction rolls and head boxes used in paper manufacturing.
In order to reduce the problems resulting when sodium dithionite decomposes to form sodium thiosulfate, the sodium dithionite is often sold in solid form. In this form it generally has a maximum purity of about 85 percent and must be dissolved in order to be useful for many processes. However, the dissolution is often performed some distance from the use site, which may allow the undesirable decomposition to occur during transport. To counter this it is alternatively possible to prepare a dithionite from aqueous sodium borohydride (NaBH.sub.4) on-site. The sodium borohydride solution is mixed with NaHSO.sub.3, which has been prepared by first reacting SO.sub.2 with NaOH. The advantage of this is that the on-site preparation reduces the time during which decomposition can take place.
Another method of reducing the decomposition of sodium dithionite to sodium thiosulfate is to use an additive, such as a chelating agent or buffer, in the sodium dithionite product. This is commonly done in paper manufacturing processes during the pulp bleaching process. Commonly used additives include, for example, ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), propylene oxide, zinc sulfate, oxalic acid, formaldehyde and formic acid. See. e.g., U.S. Pat. Nos. 3,672,829 and 3,669,895. It is alternatively possible to add sodium formate to the sodium dithionite, as disclosed in U.S. Pat. No. 4,622,216. In general these additives do not prevent all decomposition; however, because of the additives the total sodium thiosulfate/sodium dithionite ratio is less than it would be otherwise. Another method of inhibiting decomposition is disclosed in U.S. Pat. No. 3,773,679, involving the introduction of sodium sulfite or an analogue, preferably with a pH adjustment, to the sodium dithionite product under aerobic or anaerobic conditions. Mixtures of sodium sulfite or sodium bicarbonate with sodium bisulfite are also noted to be effective. In all cases the additives are introduced into the sodium dithionite product at some point following its production.
Thus, because of the undesirability of decomposition a method of producing sodium dithionite that can be carried out on-site production immediately prior to use is desired. Such a method would preferably result in a sodium dithionite product having an acceptable sodium thiosulfate/sodium dithionite ratio.